1. Field of the Invention
The present invention relates generally to operational maintenance surveillance of telecommunications equipment. More particularly, the present invention relates to a method and apparatus for measuring receive antenna return loss in a CDMA (Code Division Multiple Access) radio system.
2. Description of Related Art
Telecommunications equipment traditionally has been offered with a significant number of features allowing on-line system test and operational maintenance surveillance. These features allow economical system OAandM (Operation, Administration and Maintenance) since routine system testing and monitoring can be automatically performed and service provider technical staff can be relieved of these tasks.
After initial installation of a telecommunications system, determining the integrity of a base station antenna is an important concern. The Receive Antenna Return Loss test is a diagnostic measurement routinely automated with analog cellular base station products, which provides a reasonable verification of sustained antenna integrity. This test quantifies the reflection characteristics of an antenna in order to detect whether the antenna is functioning within desired parameters. The ratio of RF (Radio Frequency) power reflected from the antenna to the RF power applied to the antenna defines the reflection coefficient of the antenna. A reflection coefficient having a value close to zero (0) indicates that very little RF power is reflected and that the antenna is functioning properly. A reflection coefficient having a value close to one (1) indicates that most of the RF power is reflected and that the antenna is transmitting virtually zero RF power. Transmission of very low RF power indicates problems with the antenna.
Return loss is the magnitude part of the reflection coefficient expressed in decibels (dB). That is, return loss measures the magnitude of a reflected RF signal relative to the original incident RF signal. Return loss is typically referred to in terms of dBs of loss. As such, a large return loss value indicates very little reflection. For example, a return loss of 40 dB indicates very little reflection, while a return loss of 0 dB indicates complete reflection.
In analog systems, the Receive Antenna Return Loss test is performed by applying a signal from a RTU (Radio Test Unit) in the mobile receive band and monitoring the signal of a selected receive radio that is assigned to the selected channel frequency. A directional coupler is used to allow measurement of signal energy in both the forward direction and reverse direction, and a switch matrix, that is program controlled, selects the desired direction. The power difference between the forward and reverse signal levels is a measure of the return loss or impedance match accuracy of the antenna system. The forward and reverse signal levels are measured by querying the selected receive radio for its RSSI (Receive Signal Strength Indicator) output.
For a call in a CDMA system, a CDMA call, a unique code is assigned to all speech bits, and signals for all calls are spread across a broad frequency spectrum. The dispersed signals are pulled out by a receiver which knows the code for the call it must handle. This technique allows numerous phone calls to be simultaneously transmitted at one radio frequency. However, because all channels on a given carrier occupy the same frequency, a unique RSSI for one given channel is not available. As such, the conventional return loss test cannot be performed. Incremental RSSI measurements may be possible, but are confounded by the fact that typical CDMA signals are well below even the thermal noise level in most circumstances. Therefore, incremental RSSI measurements are extremely difficult and impractical.
Further, CDMA systems use closed loop mobile transmit RF power control. Achieving a known and stable RF level on a channel would violate such closed-loop power control and could confound other calls in progress on the system. Such approach would also require customized firmware and equipment for implementation.
Therefore, there is a need for an economical and practical method for measuring return loss of a receive antenna in a CDMA radio system.
The present invention uses the effects of closed loop power control in order to measure the return loss of a receive antenna in a CDMA radio system. A standard CDMA all is established under closed loop power control by a mobile phone in a test unit. Using a switch matrix, the call is directed on the incident path of a receiving antenna. After a stabilization interval, the mobile phone in the test unit is queried for its transmit power level. The established call is then directed on the reflected path of the receiving antenna. After a stabilization interval, the mobile phone is again queried for its transmit power level. The difference in dB between the reflected path transmit power level and the previous incident transmit power level, i.e., the reference transmit level taken when the call was set up, is the return loss of the antenna.